Branch-Circuit, Feeder and Service Calculations, Part XIII

Article 220—Load Calculations; 220.42 General Lighting:

Whether electricity consumers realize it or not, the National Electrical Code (NEC) is very important in their daily lives; for those of us in the electrical industry, the NEC serves a greater function. The purpose of the Codeis the practical safeguarding of people and property from hazards arizing from the use of electricity {90.1}. Before installing cable or conduit, branch circuits and feeders must be planned and calculated. While there are many rules of thumb for calculations, there are no substitutes for calculations in accordance with the National Electrical Code. NEC load calculations are a bit of a mystery to many electricians, but with the right guidance, branch-circuit, feeder and service load calculations are not complicated. is the practical safeguarding of people and property from hazards arising from the use of electricity [90.1]. Before installing cable or conduit, branch circuits and feeders must be planned and calculated. While there are many rules of thumb for calculations, there are no substitutes for calculations in accordance with the

Article 220 contains requirements for calculating branch-circuit, feeder and service loads. This article is divided into five parts. Part I contains general requirements for calculation methods. Calculation methods for branch circuits are in Part II. Part III provides requirements for feeder and service calculations. Part IV covers optional feeder and service load calculation specifications. Farm load calculation requirements are in the last part of Article 220, Part V. Last month’s Code in Focus concluded by covering general lighting load demand factors for dwelling units as specified in 220.42 and Table 220.42. This month, the analysis continues with general lighting load demand factors for hospitals.

The demand factors specified in Table 220.42 shall apply to that portion of the total branch-circuit load calculated for general illumination [220.42]. Reducing the calculated general lighting load is permissible for certain types of occupancies. The lighting loads can be reduced or lowered by applying the demand factors specified in Table 220.42. The occupancy types include dwelling units; hospitals, hotels and motels, including apartment houses without provision for cooking by tenants; and storage warehouses (see Figure 1).

Note that lighting load demand factors for dwelling units were covered last month.

If the square-foot area is known but volt-amperes for the lighting load are not, calculate the general lighting load before applying Table 220.42 demand factors. For example, a hospital has patient rooms totaling 45,000 square feet. What is the lighting load after demand factors? The general lighting load for hospitals is 2 volt-amperes per square foot. Therefore, multiply the square-foot area by 2 volt-amperes per square foot (45,000 × 2 = 90,000). The general lighting load is 90,000 volt-amperes. Multiply the first 50,000 by 40 percent (50,000 × 40% = 20,000). This leaves 40,000 volt-amperes (90,000 – 50,000 = 40,000). Multiply the remaining 40,000 volt-amperes by 20 percent (40,000 × 20% = 8,000). The general lighting load for these hospital patient rooms is 28,000 volt-amperes (20,000 + 8,000 = 28,000) (see Figure 3).

The two middle occupancy types listed in Table 220.42 include a footnote (see Figure 1). It is not permissible to apply Table 220.42 demand factors to the calculated load of feeders or services supplying areas in hospitals, hotels and motels where all the lighting is likely to be energized at the same time [Note under Table 220.42]. Examples of areas include operating rooms, ballrooms and dining rooms. In areas of hospitals, hotels and motels where the entire lighting is likely to be used at one time, continuous loads must be considered. Instead of applying demand factors to reduce lighting loads, it may be necessary to increase lighting loads. When determining conductor sizes, conductors must have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load [210.19(A)(1) and 215.2(A)(1)]. The rating of overcurrent devices must not be less than the noncontinuous load plus 125 percent of the continuous load [210.20(A) and 215.3]. For example, in determining feeder conductor sizes, what is the lighting load for an area in a hospital having operating rooms totaling 6,000 square feet and halls totaling 1,000 square feet? First, apply the unit loads of Table 220.12 to find the general lighting loads. Multiply the area of the operating rooms by 2 volt-amperes per square foot for hospitals (6,000 × 2 = 12,000). Since halls, corridors, closets and stairways have a unit load of 0.5 volt-amperes per square foot, multiply the hall area by 0.5 (1,000 × 0.5 = 500). Although this is a hospital, do not apply Table 220.42 demand factors. Because this lighting will be energized for three hours or more, the lighting load will be continuous. Multiply this lighting load by 125 percent (12,500 × 125% = 15,625). The calculated load required for sizing feeder conductors in this example is 15,625 volt-amperes (see Figure 4).

Hotels and motels are the third occupancy type listed in Table 220.42. Included with hotels and motels are apartment houses that do not have tenant cooking provisions. Where the lighting load exceeds 100,000 volt-amperes, apply all three demand factors. For example, what is the lighting load, after demand factors, for a motel that has a calculated lighting load of 120,000 volt-amperes for guest rooms? Multiply the first 20,000 volt-amperes by 50 percent (20,000 × 50% = 10,000). This leaves 100,000 volt-amperes (120,000 – 20,000 = 100,000). The demand for the next 80,000 is 32,000 volt-amperes (80,000 × 40% = 32,000). The remainder over 100,000 is multiplied by 30 percent (20,000 × 30% = 6,000). The general lighting load for the guest rooms in this motel is 48,000 volt-amperes (10,000 + 32,000 + 6,000 = 48,000) (see Figure 5).

Do not apply Table 220.42 demand factors to areas in hotels and motels where the entire lighting is likely to be used at one time.

While applying Table 220.42 demand factors to feeder and service load calculations is permissible, applying the demand factors to determine the number of branch circuits for general illumination is not permissible [220.42].

Next month’s Code in Focus continues the analysis of load calculations. EC

MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classes and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code and NFPA’s Electrical Reference. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by telephone at 615.333.3336, or via e-mail at charles@charlesRmiller.com.